Study On Heat Transfer Model Of High Efficient Heat Exchange System In Ground Source Heat Pump

The low-grade thermal energy（shallow ground temperature energy）reserved in the near-surface stratum comes from the thermal diffusion process of the core heat from the stratum to the surface.Vertical heat conduction of ground temperature is the source of shallow ground temperature energy and the key to maintain long-term and stable operation of geothermal heat pump system.Aiming at the the theoretical defect of ignorance of existing classical theories to vertical heat conduction,combining with the characteristics of efficient heat transfer holes structure,considering the actual setting of these holes at home and abroad,this paper focuses on the theoretical model of heat transfer of rock-soil body under the function of heat transfer holes and its solutions.According to the model solution,the mathematical significance of the solution is explored,and the relationship between the solution and the existing classical theoretical model solution is proved.The application method and process of model solution are demonstrated by application examples.The major research and results are sketched out as follows:（1）On condition of considering the vertical heat transfer of ground temperature,one-dimensional steady-state heat transfer model is established for the rock-soil body heat transfer under the function of a single heat transfer hole,as well,the analytical solution and application method of the model were given.Hence,a simple and feasible calculation method was provided for the hole spacing design based on the thermal response test.For the new model solution,the transformation relationship between it and the existing classical theoretical model solution was proved,and the calculation error formula of the solution of the classical theoretical model（Hart and Couvillion model）was given,also,the applicable conditions of the classical solution were clarified.In the living example,based on the classical theoretical method,the thermal response test is conducted to calculate the rock-soil thermal conductivity coefficientλs is 1.4241W/（m·K）.According to the new methodology,by using the thermal response test with two geothermal observation holes,it is calculated thatλs=1.03w/（m·K）,38%smaller than the classical method.According to new methodology and the thermal response test,it is calculated that the recharge rate obtained from vertical heat transfer during the test is 9.19 mW.m^-2(terrestrial heat flow value is about 50 mW.m^-2).The heat transfer system designed based on the calculation results has been running for 7 years.（2）Considering the fact that most heat transfer holes in China are arranged in dense linear,one-dimensional unsteady heat transfer model in homogeneous rock-soil body at the both sides of holes under vertical heat transfer function is established by taking a row of linear heat transfer holes as the first boundary of the heat source and the vertical thermal conductivity of the ground temperatureεas the source term.In the condition that the water temperature of the circulating water in the heat transfer hole is constant or stable for a long time after the instantaneous change of water temperature,εis a constant.And its application method are given by Laplace transform.Aiming at the possible changes ofε,to disperse calculation period into several ones withεis constant in each period.Based on the solution under the condition ofεis constant,solution is given by superposition principle,and recursive equation ofεin all periods.On the basis of the ground temperature dynamic data of the monitoring hole during the operation of the heat transfer system,inflected point method is established to calculate the thermal conductivity coefficient a by using the inflected point of the curve of the change velocity of the ground temperature with time.By the example,a is calculated as 5.48×10^-5m²/s.（3）When the water temperature change of heat transfer holes is a complex function（exponential form,for example）,it is limited by difficult inverse transformation functions,under the condition of free from relying on boundary function transformation process,by using specific properties of the Fourier transform and convolution,model theory solution is gained first,and then substitute the boundary function,the solution is worked out.When it is difficult to determine a unified mathematical expression for the water temperature change process,Lagrange linear discretization of the water temperature change process is carried out,and the discrete equation is substituted into the above theoretical solution,thus the solution of the practical issue is obtained.Based on the dynamic monitoring data of the ground temperature in the monitoring hole,the curve-fitting method of finding the parameters of the model,which are suitable for curve of the change velocity of the ground temperature with time and the curve of the theory is established.Using the data of different geothermal observation holes in the same example,the curve-fitting method calculates out a 5.00×10^-5m²/s,which is basically consistent with that of the inflected point method.